Platinum (Pt)-based precious metals are generally used as catalysts for oxidation/reduction reactions of fuel cells. However, the platinum-based catalysts have not been commercialized due to their high price. To solve this, research on platinum alloy catalysts or non-platinum-based catalysts has been underway. The non-platinum-based catalysts are practically inapplicable to fuel cells due to their low activity. On the other hand, the platinum alloy catalysts contain another metal with a predetermined ratio instead of platinum and have advantages of economic benefits due to decreased amount of platinum and improved catalytic activity caused by alloy effects.
Research has been actively conducted on Pt3M having a face-centered cubic structure alloyed with platinum (Pt) and a transition metal (M=Ti, V, Cr, Fe, Co, Ni) as one of platinum alloy catalysts for fuel cells. A general method of producing an alloy catalyst is co-precipitation including simultaneously reducing a platinum salt and a transition metal salt. In accordance with the co-precipitation, based on difference in reduction potential, platinum is first reduced and the transition metal is then reduced so that the alloy surface is rich in the transition metal. As a result, the transition metal is eluted under an acidic environment of the fuel cell, causing a problem such as deteriorating performance of the fuel cell. Accordingly, to solve this problem, efforts are made to produce a platinum alloy having a core-shell structure including a platinum skin layer. A general method of forming a core-shell structure is conducted using difference in solid diffusion between platinum and a transition metal based on heat treatment at a high temperature of 700 to 1200° C. However, since catalyst particles severely agglomerate and grow during the high-temperature heat treatment, effective surface area of the catalyst is decreased and the overall catalytic activity is thus deteriorated.
To solve these problems, recently, research has been underway to produce a variety of core-shell-type platinum alloy catalysts. The Adzic research team produced a platinum monolayer alloy catalyst having a core-shell structure using under potential deposition (UPD). The Strasser research team produced a platinum alloy having a core-shell structure catalyst, a transition metal of which is removed from the surface by a dealloying reaction according to an electrochemical method. The Strasser research team produced an alloy catalyst by preparing a transition metal core and depositing a platinum shell thereon. These methods enable preparation of core-shell type alloy catalysts, but all of them are disadvantageously unsuitable for mass-production because they need to control the voltage of respective particles in an electrochemical manner.
In order to solve these problem, Dr. Han-sung Kim's research team developed a method of inhibiting growth of platinum alloy catalysts during high-temperature heat treatment by introducing polypyrrole (PPy) and polydopamine (PDA) as capping materials. In accordance with this method, when a polymer protective coating is formed on a carbon supported platinum catalyst, a transition metal is impregnated in the protective coating and heat treatment is conducted, the polymer protective coating is thermally decomposed and the transition metal present in the polymer protective coating diffuses into platinum particles to form a core-shell structure. In this process, the polymer protective coating functions to suppress particle growth which may be caused by agglomeration of platinum particles. However, as high-temperature heat treatment is conducted, the polymer protective coating is gradually removed, decreasing the ability to suppress agglomeration. When growth of particles is not sufficiently suppressed, the problem of size unbalance between particles remains.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.